Carbon nanotubes (CNTs), comprising multiple concentric shells and termed multi-wall carbon nanotubes (MWNTs), were discovered by Iijima in 1991 [Iijima, S. Nature 1991, 354, 56]. Subsequent to this discovery, single-wall carbon nanotubes (SWNTs), comprising a single graphene rolled up on itself, were synthesized in an arcdischarge process using carbon electrodes doped with transition metals [Iijima, S.; Ichihashi, T. Nature 1993, 363, 603; and Bethune, D. S., Kiang, C. H.; de Vries, M. S.; Gorman, G.; Savoy, R.; Vasquez, J; Beyers, R. Nature 1993, 363, 605]. These carbon nanotubes (especially SWNTs) posses unique mechanical, electrical, and thermal properties, and such properties make them attractive for a wide variety of applications.
With extremely high modulus, strength, flexibility and lightweight, carbon nanotubes are seen as excellent reinforcement candidates for developing next generation advanced composites materials [Calvert, P. Nature, 1999, 399, 210; Thostenson, Composite Science and Technology. 2001, 61, 1899-1912; Maruyama, B., Alam, K. SAMPE Journal, “Carbon Nanotubes and Nanofibers in Composites Materials,” 2002, 38 (3), 59-70]. Expected property enhancements for composites include: enhanced strength and stiffness, improved toughness and shear strength, other Z-axis properties, and improved electrical and thermal conductivity. Significant efforts are being made to advance polymer nanocomposites by the addition of carbon nanotubes. However, it has been realized that, in order to take advantage of the extraordinary properties of nanotubes, an effective load transfer from the matrix to nanotubes must exist [L. S. Schadler, S. C Giannaris, P. M. Ajayan, “Load Transfer in Carbon Nanotube Epoxy Composites,” Appl. Phys. Lett., 1998, 73 (26), 3842-3844; P. M. Ajayan and L. S. Schadler, “Single-walled carbon nanotube type-Polymer Composite: Strength & Weakness,” Adv. Mater., 2000, 12 (10), 750-753; Lau, K. T., “Effectiveness of using carbon nanotubes as nano-reinforcements for advanced composite structures,” Carbon, 2002, 40, 1605-6]. So far, due to weak interaction between nanotubes and the polymer, the reinforcing role of nanotubes in composites is still very limited. Homogenous dispersion of nanotubes is also a necessary condition for producing an optimal reinforcing effect. Nanotubes tend to exist as bundles and are entangled as agglomerates, resulting in a poor dispersion within the polymer matrix. Although van der Waals forces might provide some physical interaction, optimal load transfer should be obtained from the matrix to the smaller bundles and, ideally, to the individual nanotubes. Therefore, unroping (debundling) the nanotubes for homogeneous dispersion remains a challenge for composites applications.
Chemical manipulation of single-wall carbon nanotubes (SWNT), especially sidewall functionalization, has recently become an area of escalated fundamental and technological interest—particularly for effecting the debundling of such SWNTs. Both covalent and noncovalent sidewall chemistry of SWNTs have been reported, including direct fluorination and subsequent derivatization, addition of radicals, carbenes and nitrenes as well as the 1,3-dipolar and electrophilic additions, and modification through van der Waals interactions with aromatic molecules or polymers. See Khabashesku, V. N.; Margrave, J. L. “Chemistry of Carbon Nanotubes” in Encyclopedia of Nanoscience and Nanotechnology, Ed. S. Nalwa, American Scientific Publishers, 2004, Vol. 1, pp. 849-861, and references therein; Khabashesku, V. N.; Billups, W. E.; Margrave, J. L. Acc. Chem. Res., 2002, 35, 1087; Bahr, J. L.; Tour, J. M. J. Mater. Chem. 2002, 12, 1952; Georgakilas, V. et al., “Organic Functionalization of Carbon Nanotubes,” J. Am. Chem. Soc., 2002, 124 (5), 760-761. The applications of functionalized SWNTs as reinforcers for fabrication of covalently integrated polymer composites [Barrera, E. V. JOM, 2000, 52, 38; Zhu, J.: Kim, J.; Peng, H.; Margrave, J. L.; Khabashesku, V. N.; Barrera, E. V. Nano Lett. 2003, 3, 1107; Zhu, J.; Peng, H.; Rodriguez-Macias, F.; Margrave, J. L.; Khabashesku, V. N.; Imam, M. A.; Lozano, K.; Barrera, E. V. Adv. Funct. Mater, 2004, 14(7), 643-648] and as vehicles for targeted drug delivery have recently been demonstrated. See Pantarotto, D.; Partidos, C. D.; Graff, R.; Hoebeke, J.; Briand, J.-P.; Prato, M.; Bianco, A. J. Am. Chem. Soc. 2003, 125, 6160. Indeed, these studies have confirmed the need for derivatization of the SWNTs with the organic functional groups which can provide a high binding affinity and selectivity through covalent or hydrogen bond formation. They also suggest that for processability improvement, particularly in biomedical applications, covalent sidewall functionalization with moieties terminated with hydrophilic substituents, such as hydroxyl groups, should be of primary importance.
Recent experimental studies [Khabashesku, V. N.; Billups, W. E.; Margrave, J. L. Acc. Chem. Res., 2002, 35, 1087] have shown that fluoronanotubes prepared by direct fluorination of SWNTs can be used as a versatile precursors for preparation of sidewall functionalized nanotube derivatives through a nucleophilic substitution of fluorine. A simple method for introducing hydroxyl functionalities to CNTs, and especially SWNTs, utilizing fluorinated carbon nanotubes as intermediates has also been demonstrated, permitting the dispersal of carbon nanotubes in polar solvents [L. Zhang et al., Chem. Mater. 2004, 16, 2055-2061]. Such functionalized CNTs are termed “hydroxyl-functionalized CNTs” or “hydroxyl-functionalized nanotubes” herein.
Recent research has shown a great deal of potential for the use of functionalized nanotubes as reinforcement in composites, wherein such reinforcement is derived primarily from the improved load transfer stemming from improved interaction and interfacial bonding [J. Zhu, H. Peng, F. Rodriguez-Macias, J. L. Margrave, V. N. Khabashesku, A. M. Imam, K. Lozano, E. V. Barrera, Adv. Fun. Mater., 2004, 14(7), 643-648] Specifically, if such hydroxl-functionalized CNTs could be used for subsequent reactions to introduce specific polymerizable or crosslinkable moieties covalently onto the CNTs, such functionalized CNTs could be used to form high-performance composites with a wide variety of polymer systems.